The method and system disclosed herein, in general, relates to information communication. More particularly, the method and system disclosed herein relates to communicating distinct data over a single frequency using multiple signals under at least two different polarization schemes.
Current satellites and ground based microwave radios, typically reuse frequencies by transmitting signals in two polarities of one of two polarization schemes: left and right hand circular polarization, or vertical and horizontal linear polarization. Normally, if different data signals are transmitted on the same frequency in both linear and circular polarizations, the data signals would interfere with each other. Consequently, no more than two different data signals are transmitted on the same frequency at an instant, representing the two polarities of a selected polarization scheme.
Another form of frequency reuse is through separation. Additional channels can be transmitted on the same frequencies as long as there is sufficient distance between the transmitters so that antennas can pick up the selected transmissions with minimal interference. In the case of satellites, the satellites must maintain a distance of about 2 degrees of arc before the same frequencies can be reused. In the case of televisions, terrestrial microwave radios, and radio stations, geographic distance is used to ensure sufficient distance or attenuation between the transmitters.
In linear polarization, the electric component or the magnetic component of an electromagnetic wave is confined to within a single plane along the direction of propagation of the electromagnetic wave. Linearly polarized signals are either horizontally linearly polarized or vertically linearly polarized. In circular polarization, the tip of the electric field vector is made to describe a circle as time passes. Circularly polarized data signals are either right hand circularly polarized or left hand circularly polarized. Elliptical polarization is similar to circular polarization, except that the angle vector changes in a non linear fashion. Polarity can be established by either the shape of the radiation elements in the case of a lower frequency antenna, for example frequency modulated (FM) radio antenna, or by a feed horn often feeding a larger, usually parabolic reflector in a higher frequency band. The different polarization schemes and this disclosure apply to any frequency electromagnetic waves that can be polarized including, for example, light, microwave, and radio frequency waves.
Linearly polarized data signals and circularly polarized data signals can be propagated in different fashions. In lower frequency signals, the shape of the antenna can determine the shape of the signal. In higher frequency signals having a frequency above 2 to 3 gigahertz (GHz), usually a feed horn and at least one reflector are typically used. In even higher frequencies, emitters and filters are used.
As used herein, the term “feed horn” refers to an apparatus that includes both a horn and a transducer, also called a polarizer. The transducer polarizes the signal for transmission. A transducer is a mechanical device that bolts to the back of the horn and shapes and transmits the signal, as well as picks up already polarized data signals for reception. A transducer also routes the data signals from a transmission side of input flanges to the horn or from the horn to a reception side of output flanges. A transducer can form or receive signals of linear, circular or both polarities. Elliptical transmissions are rarely used for communications, and for purposes of this disclosure, shall be treated as equivalent to circular transmissions.
Left hand circularly polarized data signals and right hand circularly polarized data signals interfere with each other only to an insignificant degree when transmitted together. Similarly, horizontal and vertical linearly polarized data signals basically do not interfere with each other when transmitted together. However, each pole of a linear feed and transducer, whether horizontal or vertical, picks up both left hand circularly polarized signals and right hand circularly polarized signals simultaneously in almost equal levels of about 3 decibels (dB) less than, therefore half of, the full strength of a correctly aligned circularly polarized feed. Similarly, an antenna or feed of circular polarity picks up the horizontal and vertical linearly polarized signals in both left and right circular polarities at about half the level of what a correctly aligned linear feed would. Along the axis of transmission, the rotation or angle of the linear receive feed in relation to the circular polarized transmission feed does not affect the reception level of circularly polarized signals in the linear feed. Similarly, each pole of a circular feed and transducer, whether left hand circular or right hand circular, typically picks up both horizontally polarized signals and vertically polarized signals in almost equal levels.
On any given frequency, normally attempting to transmit both linearly polarized signals and circularly polarized signals simultaneously results in so much interference in a receive antenna that the received signal is not usable. Linearly polarized signals suffer from interference from circularly polarized signals, whereas circularly polarized signals suffer from interference from linearly polarized signals.
Until now, due to the noise and interference involved, only a single polarization scheme can typically be used to communicate distinct data signals on the same frequency. This means on a given frequency a maximum of two data signals can be transmitted simultaneously, one on each polarity of the chosen polarization scheme. There is a need for transmitting an additional data signal along the same or proximate path resulting in about a 50% to about a 100% increase in capacity. Hence, there is an unmet need for communicating additional distinct data over a single frequency using multiple polarized data signals.